The efficient Doherty power amplifier DPA has become increasingly popular in communication systems. However, the DPA has defective properties such as very narrow bandwidth and large size. The DPA narrow-band property is mainly due to the narrow-band property of a narrow-band mixer. An output mixer generally has a higher Q point, while the higher Q point causes a narrower bandwidth. With a balance-type DPA as an example, the Q point may amount to 0.76. However, the large-size property is due to the fact that the DPA output mixer uses two ¼ wavelength lines (λ/4). Particularly for a 900 MHz system, when using Rogers's high-frequency printed circuit board material RO4350B as the PCB material, the length of the ¼ wavelength line is about 47 mm; therefore, the size design cannot be scaled down.
Currently, a scheme for extending bandwidth for a DPA has been proposed, as shown in FIG. 1. In that scheme, a ¼ wavelength line 101 of 35.3 Ohm is grounded to form a minor with the ¼ wavelength line of the Doherty mixer on a layout. When the DPA works at a relatively lower input power, the peaking amplifier is closed, and the carrier amplifier will work in a high-resistance state (generally at 100 Ohm). For the DPA having the 35.3 Ohm ¼ wavelength line 101, resistance dispersion will become more converged than that of a common DPA with the carrier amplifier working at the high-impedance. Therefore, the bandwidth of a bandwidth-extended DPA will be much wider than that of a non-bandwidth extended DPA, as shown in FIG. 2.
However, at present, small-size design has become a fashion. Although the above scheme provides an idea to design a wideband DPA, the additional 35.3 Ohm ¼ wavelength line occupies more space. It becomes a problem for the PCB (printed circuit board) design of a power amplifier.